Jay Gatsby gets most of his wealth from his illegal business in liquor smuggling. This book is set during the Prohibition era in the United States (1919-1933). During this time, it was illegal to produce, transport, or sell alcoholic beverages. Because of this, and because people still wanted to drink, there was a huge black market for alcohol.
In real life, many bootleggers got rich off of this illegal trade. In the book, Gatsby gets rich in this same fashion. You can get confirmation of this, for example, in Chapter 7. Tom accuses Gatsby and Wolfsheim of having been bootleggers and Gatsby does not deny it.
He and this Wolfsheim bought up a lot of side-street drug-stores here and in Chicago and sold grain alcohol over the counter. That’s one of his little stunts. I picked him for a bootlegger the first time I saw him, and I wasn’t far wrong.”
As was mentioned in the previous post, Jay Gatsby earns his fortune through the illegal sale and distribution of liquor. The Prohibition Act of 1919 banned the production, importation, and distribution of alcohol in the United States. The Prohibition Era lasted until 1933, and many Americans illegally purchased their alcohol from bootleggers. In the novel The Great Gatsby, Jay Gatsby hides the fact that he is a wealthy bootlegger. In Chapter 4, Nick Carraway hears the rumor that Gatsby made his fortune bootlegging. Throughout the novel, Nick is introduced to Mr. Wolfshiem, Gatsby's shady business partner, and Gatsby even offers Nick an opportunity to join his "business." In Chapter 7, Tom Buchanan confronts Gatsby about his illegal bootlegging business in front of Daisy. Tom even mentions that Walter Chase could also have Gatsby indicted on illegal gambling laws, and Gatsby denies Tom's allegations. However, Daisy believes her husband and realizes that being with Jay Gatsby is too risky. In Daisy's mind, it is only a matter of time before Gatsby goes to jail.